<!DOCTYPE html>
<html>
<body>
<select>
  <option>Apple</option>
  <option>Orange</option>
  <option>Banana</option>
</select>
<script>
  document.querySelector('select').focus();
  function go() {
    var comboBox = document.querySelector('select');
    var clickEvent = document.createEvent('MouseEvents');
    clickEvent.initMouseEvent('mousedown', true, true, window);
    comboBox.dispatchEvent(clickEvent);
  }
</script>
</body>
</html>
